An APZ (Automatic Program Zooming) is already known in which a zoom lens of a camera is automatically driven (i.e., the focal length of the zoom lens is adjusted) to take a picture of a main object (e.g., a man) so that the size of the main object in the picture frame is adjusted. Normally, the APZ is performed by the following process. First, the distance D to the main object is detected, and then a necessary magnification .beta. for projecting the main object at the distance D onto a film with a predetermined size is determined using a line of a previously prepared diagram. There may be several lines on a diagram for different purported sizes in a picture frame (e.g., for full portrait and bust portrait). The target focal length of the zoom lens is calculated by: EQU f=.beta..multidot.D, (1)
and the zoom motor is driven to bring the zoom lens at the focal length f.
Another prior art relating to the present invention is the detection of a photographer looking into a viewfinder. Japanese Published Unexamined Patent Application No. S52-110037 shows a camera having a thermosensor for sensing an access by a photographer. The thermosensor is placed near the viewfinder, and when a photographer looks into the viewfinder, the thermosensor senses his body heat. When the sensor detects the heat, power is supplied to the focus condition detector of the camera. A problem about this human body heat detection is that it does not always work correctly. When some heat radiation comes into the thermosensor near the viewfinder from behind the camera, unnecessary focus detection is performed.
Japanese Published Unexamined Patent Application No. S64-42639 discloses a camera in which: a focus condition detecting operation is started when a photographer is detected to take hold of a grip portion of the camera, and the auto-focusing lens is driven when the photographer is detected to look into the viewfinder. A problem of this camera is that a focus condition detecting operation is started before he looks into the viewfinder. Because the picture frame cannot be decided before the photographer looks into the viewfinder, the detecting operation is useless. Also, the battery power will be wasted while he simply holds the camera without an intention of taking a picture.